Arrayed waveguide grating

ABSTRACT

An arrayed waveguide grating (AWG) device ( 1 ) comprising a substrate having a first array of waveguide ( 8 ) optically coupled between first and second slab couplers ( 3,4 ) and a second array of input-output waveguides ( 10 ) optically coupled at first ends thereof to an input/output side ( 5 ) of the second slap-coupler ( 43 ). The input/output waveguide are tapered at first end portions ( 13 ) thereof so as to increase in width towards the second slap coupler, and the width (W) of the first ends of the input/output waveguides varies across the second array ( 10 ) in a manner so as to increase uniformity of at least on performance parameter, for example adjacent crosstalk (AXT). In the described embodiment, the width (W) of the tapered waveguide ends increases from one side to the array of input/output waveguides ( 10 ) to the other, so as to keep the separation (s) between the input/output waveguides

[0001] The present invention relates to an improved arrayed waveguidegrating (AWG) device.

[0002] In order to meet the ever-increasing demand for transmissionbandwidth in communication networks, operators are investing heavily inthe development of techniques for Dense Wavelength Division Multiplexing(DWDM). DWDM employs many closely spaced carrier wavelengths,multiplexed together onto a single waveguide such as an optical fibre.The carrier wavelengths are spaced apart by as little as 50 GHz in aspacing arrangement designed for an ITU (InternationalTelecommunications Union) channel “grid”. Each carrier wavelength may bemodulated to provide a respective data transmission channel, By usingmany channels, the data rate of each channel can be kept down to amanageable level.

[0003] Clearly, to utilize this available bandwidth it is necessary tobe able to separate, or demultiplex, each channel at a receiver. Newoptical components for doing this have been designed for this purpose,one of these being the Arrayed Waveguide Grating (AWG). An AWG is aplanar structure comprising a number of array waveguides which togetheract like a diffraction grating in a spectrometer. AWGs can be used asmultiplexers and as demultiplexers, and a single AWG design can commonlybe used both as a multiplexer and demultiplexer. A typical AWG mux/demux1 is illustrated in FIGS. 1 and 2 and comprises a substrate or “die” 1having provided thereon at least one input waveguide 2 for a multiplexedinput signal, two “slab” or “star” couplers 3,4 connected to either endof an arrayed waveguide grating consisting of an array of transmissionwaveguides 8, only some of which are shown, and a plurality of singlemode output waveguides 10 (only some shown) for outputting respectivewavelength channel outputs from the second (output) slab coupler 4 tothe edge 12 of the die 1. In generally known manner, there is a constantpredetermined optical path length difference between the lengths ofadjacent waveguides 8 in the array which determines the position of tiewavelength output channels on the output face 5 of the second slabcoupler 4. Typically, the physical length of the waveguides increasesincrementally by the same amount, ΔL, from one waveguide to the next,where

ΔL=mλ _(c) /n _(c)

[0004] where λ_(c) is the central wavelength of the grating, n_(c) isthe effective refractive index of the array waveguides, and m is aninteger number. In known manner, the waveguides and slab couplers aretypically formed as “cores” on a silicon substrate (an oxide layer iscommonly provided on the substrate prior to forming the cores thereon)and are covered in a cladding material, this being done for example byFlame Hydrolysis Deposition (FHD) or Chemical Vapour Deposition (CVD)fabrication processes.

[0005] The construction and operation of such AWGs is well known in thearts See for example, “PHASAR-based WDM-Devices: Principles, Design andApplications”, M K Smit, IEEM Journal of Selected Topics in QuantumElectronics Vol.2, No.2, June 1996. Commonly, the end portions 13 of theoutput and/or input waveguides may be adiabatically tapered in widthwhere they are coupled to the respective slab coupler 3,4, so as towiden towards the coupler, as illustrated in FIG. 2. (Only four of theoutput waveguides 10 are shown FIG. 2, for clarity). This tends toimprove the overall performance of the device, largely due to bettermode coupling between the slab couplers and the input/output waveguides.In other embodiments there are no input waveguides: instead, the firstslab coupler 3 is arranged at the edge of the die 1, so that an inputsignal can be launched directly into the slab.

[0006] One problem with AWGs is that of keeping crosstalk betweenadjacent channels as low as possible. Such “adjacent crosstalk”,hereinafter referred to as AXT, is detrimental to the performance of theAWG device. Commonly, in the above-described AWG devices, the spacing or“pitch” between adjacent ones of the output waveguides (i.e. fromoptical axis to optical axis) is not uniform across all the outputwaveguides, In fact, the pitch is usually gradually increased from thelowest frequency (channel) output waveguide to the highest frequency(channel) output waveguide. This is because the positions of thedifferent wavelengths imaged on the output face 5 of the output slabcoupler are not uniformly spaced, this being due to the non-linearity ofthe function which defines the positions of the is channel wavelengthson the output face of the slab coupler. It is highly desirable to theusers of such AWG devices (e.g. network providers) for the AWG to have aconstant channel spacing in the frequency domain. Designers thereforecommonly “chirp” the pitch of the output waveguides according to thepredicted positions of the desired output channel signals on the outputface 5 of the output slab coupler, in order that the output channels areequally spaced in the frequency domain.

[0007] In the above-mentioned common case where tapers are provided onthe ends of the output waveguides which are coupled to the output slabcoupler, the tapers are usually all identical in shape and size, thetaper width adjacent the output slab being designed for the centralchannel of the output waveguide array. Where the pitch between suchtapered output waveguides is chirped across the output waveguide array,for example the pitch may vary from 19.4 μm for channel 1 to 18.6 μm forchannel 40 in a 40 channel AWG, we have found experimentally that thereis a deviation of the AXT from the “designed for value” of AXT, acrossthe array of output waveguides. We have found that there is anapproximately linear dependence of actual AXT with channel number. Thus,the AXT for the lowest frequency channel (channel 1) will be lowest(i.e. best), while the AXT of the highest frequency channel (channel 40)will be highest (i.e. worst). As the overall AXT value for the AWG isdetermined by the worst-case value, this results in an increased ART ascompared with the designed value.

[0008] We believe this slope in the AXT value is related to the chirpingof the output waveguide pitch which is used to obtain a constant channelspacing in the frequency domain.

[0009] The chirped pitch between output waveguides can also lead tonon-uniformity in other performance parameters of the AWG, across theoutput channels. For example, non-uniformity in one or more of: thechannel bandwidth (BW); Polarisation dependent loss (PDL); insertionloss (IL); passband uniformity (PBU).

[0010] An aim of the present invention is to avoid or minimize one ormore of the foregoing disadvantages.

[0011] According to a first aspect of the invention there is provided anarrayed waveguide grating (AWG) device comprising:

[0012] a substrate having first and second slab couplers;

[0013] a first array of waveguides optically coupled between the firstand second slab couplers and having respective predetermined opticalpath length differences therebetween;

[0014] a second array of waveguides optically coupled at first endsthereof to an input/output side of the second slab coupler, thewaveguides of the second array being substantially tapered at first endportions thereof so as to increase in width towards the second slabcoupler; wherein

[0015] the width of the first ends of the waveguides varies across thesecond array in a manner so as to increase uniformity of at least oneperformance parameter across the second array of waveguides.

[0016] Most preferably, the width of the first ends of the waveguidesvaries across the second array in a manner so as to substantiallyequalize at least one performance parameter across the second array ofwaveguides.

[0017] Preferably, the width of the first ends of the waveguides variesacross the second array in a manner so as to substantially equalizeadjacent crosstalk (AXT) across the second array of waveguides.Preferably, the pitch between adjacent waveguides of the second arrayvaries across the second array and the width of said first ends of thewaveguides preferably varies across the second array so as to keep theseparation between said first ends substantially constant, wherebyvariation in AXT between the waveguides of the second array caused bysaid variation in pitch thereof is substantially compensated for.

[0018] AXT is the most pronounced performance parameter affected byvariation of the pitch of the output waveguide. Thus, the invention hasthe advantage of allowing a reduction of the worst-case AXT to beachieved, as compared with the worst-case AXT of the described priordevices in which all the tapers on the output waveguides are ofidentical width.

[0019] Alternatively, or additionally, the width of the first ends ofthe waveguides may vary across the second array in a manner so as tosubstantially equalize, or at least increase uniformity in, at least oneother performance parameter across the second array of waveguides, forexample one or more of BW, PDL, L and PBU.

[0020] It will be appreciated that the waveguides of the second arraywill be used as output or input waveguides, depending on whether the AWGdevice is used as a demultiplexer or a multiplexer respectively.Preferably, each waveguide of the second array (hereinafter referred toas “each input/output waveguide”) is adiabatically tapered at the firstend portion thereof. Alternatively, other forms of taper in which thewidth of tile waveguide increases towards the slab coupler are possible,for example parabolic tapers (sometimes referred to as “parabolichorns”) as described in JP 09297228A. In other possible embodiments theinput/output waveguides may be substantially tapered in the sense thatthey widen towards the slab coupler in non-continuous fashion. Forexample there may be an MMI (Multi-Mode Interference) device provided inthe first end of each input/output waveguide, coupled to the second slabcoupler, such as described in U.S. Pat. No. 5 629 992. In anotherpossibility a Y-branch may be provided in the first end of eachinput/output waveguide, coupled to the second slab coupler, as describedin U.S. Pat. No. 5,412,744. In all of these described embodiments, inaccordance with the invention the width of the first ends of theinput/output waveguides, optically coupled to the input/output side ofthe second slab, is varied across the array of input/output waveguidesso as to substantially equalize, or at least increase uniformity in, atleast one performance parameter, for example AXT, across the secondarray of waveguides.

[0021] Preferably, the width of the first ends of the input/outputwaveguides increases from one side of the second waveguide array to theother. Most preferably, the pitch between adjacent input/outputwaveguides decreases from the lowest frequency channel input/outputwaveguide to the highest frequency channel input/output waveguide, andthe width of the first ends of the input/output waveguides decreasesaccordingly from said lowest to said highest frequency channelinput/output waveguide, so as to substantially compensate for variationin AXT caused by the variation in pitch of adjacent input/outputwaveguides.

[0022] Preferably, the output waveguides are all single-mode, orsubstantially single-mode, waveguides.

[0023] The AWG device may further include at least one input/outputwaveguide optically coupled at a first end thereof to an input/outputside of the first slab coupler. An array of such input/output waveguidesmay be optically coupled at first ends thereof to the input/output sideof the first slab coupler, the pitch of these input/output waveguidesbeing varied across the array, and the waveguides being substantiallytapered at first end portions thereof, in which case the width of saidfirst ends thereof may also be varied so as to substantially equalize,or at least improve uniformity in, at least one performance parameteracross all the input waveguides. This may be advantageous where the AWGdevice is, for example, to be used as a router.

[0024] According to another aspect of the invention there is provided amethod of designing an AWG device comprising the steps of;

[0025] providing a substrate;

[0026] providing on the substrate first and second slab couplers, afirst array of waveguides optically coupled between the first and secondslab couplers and having respective predetermined optical path lengthdifferences therebetween, and a second array of waveguides opticallycoupled at first ends thereof to an input/output side of the second slabcoupler,

[0027] wherein the waveguides of the second array are substantiallytapered at first end portions thereof so as to increase in width towardsthe second slab coupler, and the width of said first ends of thewaveguides is varied across the second array so as to increaseuniformity of, and preferably substantially equalize, at least oneperformance parameter, for example AXT, across the second array ofwaveguides.

[0028] The pitch between adjacent waveguides of the second array mayalso be varied across the second array, in which case the width of saidfirst ends of the waveguides is preferably varied across the secondarray so as to substantially equalize AXT across the second array ofwaveguides. This may be substantially achieved by keeping the separationbetween said first ends substantially constant, whereby variation in AXTbetween the waveguides of the second array caused by said variation inpitch thereof is substantially compensated for.

[0029] According to a further aspect of the invention there is provideda multiplexer/demultiplexer comprising an AWG as above-described.

[0030] According to another aspect of the invention there is provided acommunications system incorporating at least one AWG device asabove-described.

[0031] Preferred embodiments of the invention will now be described byway of example only and with reference to the accompanying drawings inwhich:

[0032]FIG. 1 is a schematic plan view of a known AWG device;

[0033]FIG. 2 is a magnified view of the ringed portion A of FIG. 1;

[0034]FIG. 3 is a graph illustrating AXT against AWG channel number; and

[0035]FIG. 4 illustrates schematically three output waveguides of an AWGaccording to the invention.

[0036] The sloped graph G1 in FIG. 3 illustrates the variation incrosstalk which we have found in a 40 channel AWG of the typeillustrated in FIGS. 1 and 2, in which the output waveguide pitch (d)increases across the output waveguide array and the output waveguides 10are each identically tapered at the end portions 12 thereof which arecoupled to the second slab coupler 4. As shown, the AXT increasesgenerally linearly from the lowest frequency output channel (channel 1)up to the highest frequency channel (i.e. channel 40 in a 40 channelAWG), the variation in AXT in the example illustrated in FIG. 3 beingfrom about −25 dB to about −22 dB.

[0037]FIG. 4 illustrates one embodiment of the invention in which theseparation (s) between adjacent tapered ends 20 of the output waveguides10, at the second slab coupler 4, is kept constant rather then keepingthe width (W) of the tapered ends constant as in the prior at devices.As illustrated in FIG. 4, the pitch (d₁, d₂, . . . ) between adjacentoutput waveguides 10 is still varied in known manner, generallyincreasing from the highest frequency channel (channel 40) down to thelowest frequency channel (channel 1), but the width (W₁,W₂, W₃, . . . )of the adiabatic tapers, where they are connected to the second slabcoupler 4, is increased with the increasing pitch (d), so that theseparation (s) between the ends (at the second slab coupler 4) is keptconstant.

[0038] We have carried out BPM (Beam Propagation Method) simulationswhich support this proposed new configurations and arrangement of thetapers on the output waveguides.

BPM Simulation Result

[0039] The specifications were calculated for the central and outerchannels (channels 1, 20 and 40). From the results, the “slope” in AXTacross the receiver (i.e. output) waveguides, namely the differencebetween minimum and maximum AXT, and the worst-case AXT “spec” valuewere determined. The calculations were done firstly for a fixed taperwidth (W) of 14 μm and secondly for a fixed separation (s) betweentapered ends (at the output slab) of 5 μm (i.e. taper width W varyingfrom 14.4 to 13.6 μm from channel 1 to 40). The results are shown below:AXT [dB] Slope Spec Fixed Taper width 1.24 −26.7 Varied Taper Width,0.18 −27.2 fixed separation (s)

[0040] From these results, it is clear that varying the taper width (W)reduces the AXT slope across the channels: the slope reduces from 1.24dB to only 0.18 dB. This approaches the ideal “flat line” graph of AXTillustrated by plot line G2 in FIG. 3. At the same time, the AXT “spec”(i.e. worst case) value reduces (i.e. improves) by about 0.5 dB.

Conclusion

[0041] The slope in AXT can be compensated for by adjusting the width ofthe tapered ends of the output waveguides so as to equalize the AXTacross all the output waveguides. One simple way of substantiallyachieving this in the case where the pitch of the output waveguidevaries across the array is to vary the taper width (W) so as to keep theseparation (s) between the taper ends constant. BPM calculations showthat (overall) AXT is improved by around 0.5 dB, while the other specsare hardly affected

[0042] It will be appreciated that the above-described AWG, althoughdescribed with reference to its use as a demultiplexer, could equally beused as a multiplexer. Therefore, the terms “input” and “output” as usedabove are not intended to be limiting, and would be interchanged wherethe device is used as a multiplexer.

[0043] Further modifications and variations to the above describedembodiments are possible without departing from the scope of theinvention. For example, in some cases the use of the input waveguide 2may not be necessary. Instead, the input (multiplexed) optical signalmay be input directly to the first slab coupler 3. In other cases, forexample where the AWG is intended to be used as a router and thereforehas multiple input waveguides 2, the input waveguides may have similarlytapered ends (at the first slab coupler) in which the width of thetapers is also varied so as to substantially equalise AXT across theinput waveguides. For example, if the input waveguides 2 are designed sothat the pitch (d) therebetween varies then the width of their taperedends can also be varied so as to keep the separation (s) between thetapered ends constant

[0044] It will be appreciated that the above described embodiment, inwhich the separation (s) between tapered ends is kept constant, is justone example of the invention. In other possible embodiments furtheradjustment of the output Waveguide taper width (W) may be necessary inorder to compensate for variation in AXT across the output waveguidescaused by factors other than variation in pitch thereof. In such casesthe designer will adjust the individual taper widths as necessary inorder to equalize, or substantially equalize, AXT across all the outputwaveguides.

[0045] Also, in other possible embodiments of the invention the widths(W) of the tapers on the output waveguides are varied appropriately soas to substantially equalize, or at least achieve more uniformity in,one or more other performance parameters of the AWG, such as Bandwidth,PDL, insertion loss and/or passband uniformity, as already mentioned.Additionally, if desired the tapered input waveguides (where provided)may also have the widths of their tapered ends adjusted so as tosubstantially equalize, or at least improve uniformity in, at least oneor more performance parameters of the AWG across the input waveguidesthereof. This may be useful where, for example, the AWG device will beused as a router.

1. An arrayed waveguide grating (AWG) device comprising: a substratehaving fist and second slab couplers; a first array of waveguidesoptically coupled between the first and second slab couplers and havingrespective predetermined optical path length differences therebetween; asecond array of waveguides optically coupled at first ends thereof to aninput/output side of the second slab coupler, the waveguides of thesecond array being substantially tapered at first end portions thereofso as to increase in width towards the second slab coupler; wherein thepitch between adjacent waveguides of the second array varies across thesecond array and the width of said first ends of the waveguides variesacross the second array so as to keep the distances between adjacentedges of adjacent waveguides at the first end in the directionperpendicular to the waveguide axis substantially constant so as toincrease uniformity of at least one performance parameter across thesecond array of waveguides.
 2. An AWG device according to claim 1,wherein the width of the first ends of the waveguides varies across thesecond array in a manner so as to substantially equalize at least oneperformance parameter across the second array of waveguides.
 3. An AWGdevice according to claim 1 or claim 2, wherein the width of the firstends of the waveguides varies across the second array in a manner so asto substantially equalize adjacent crosstalk (AXT) across the secondarray of waveguides.
 4. An AWG device according to claim 3, wherein thewidth of said first ends of the waveguides increases from one side ofthe second waveguide army to the other.
 5. An AWG device according toclaim 4, wherein the pitch between adjacent waveguides of the secondarray decreases from the lowest frequency channel waveguide to thehighest frequency channel waveguide, and the width of said first ends ofthe waveguides decreases accordingly from said lowest to said highestfrequency channel waveguide, so as to substantially compensate forvariation in AXT caused by the variation in pitch of adjacent waveguidesof the second array.
 6. An AWG device according to claim 1 or claim 2,wherein said at least one performance parameter is selected from thefollowing parameters adjacent crosstalk (AXT), bandwidth (BW),Polarisation Dependent Loss, (PDL), Insertion Loss (IL), PassbandUniformity (PBU).
 7. An AWG device according to any preceding claim,each waveguide of the second array is tapered adiabatically at the firstend portion thereof.
 8. An AWG according to any of claims 1 to 6,wherein the input/output waveguides widen towards the second slabcoupler in non-continuous fashion.
 9. An AWG device according to anypreceding claim wherein the output waveguides are all substantiallysingle-mode waveguides.
 10. An AWG device according to any precedingclaim, wherein the AWG device further includes at least one input/outputwaveguide optically coupled at a first end thereof to an input/outputside of the first slab coupler.
 11. An AWG device according to claim 10,wherein an array of input/output waveguides are optically coupled atfirst ends thereof to the input/output side of the first slab coupler,the pitch of these input/output waveguides being varied across thearray, and the waveguides being substantially tapered at first endportions thereof, and wherein the width of said first ends thereof isalso varied so as to improve uniformity in at least one performanceparameter across all the input/output waveguides.
 12. A method ofdesigning an AWG device comprising the steps of: providing a substrate;providing on the substrate first and second slab couplers, a first arrayof waveguides optically coupled between the first and second slabcouplers and having respective predetermined optical path lengthdifferences therebetween, and a second array of waveguides opticallycoupled at first ends thereof to an input/output side of the second slabcoupler, the waveguides of the second array being substantially taperedat first end portions thereof so as to increase in width towards thesecond slab coupler, wherein the pitch between adjacent waveguides ofthe second array varies across the second array and the width of saidfirst ends of the waveguides varies across the second array so as tokeep the distances between adjacent edges of adjacent waveguides at thefirst end in the direction perpendicular to the waveguide axissubstantially constant so as to increase uniformity of at least oneperformance parameter across the second array of waveguides.
 13. Amethod according to claim 12, wherein said at least one performanceparameter is selected from AXT, BW, PDL, IL PBU.
 14. A method accordingto claim 12, wherein the pitch between adjacent waveguides of the secondarray is also varied across the second array, and the width of saidfirst ends of the waveguides is varied across the second array so as tosubstantially equalize AXT across the second array of waveguides.
 15. Amultiple/demultiplexer comprising an AWG device according to any ofclaims 1 to
 12. 16. A communications system incorporating at least oneAWG device according to any of claims 1 to 12.